1. Field of the Invention
This invention relates to improved precipitated silica compositions. More particularly, the present invention relates to improved precipitated silica compositions that impart improved cleaning and abrasive performance to dentifrice formulations. Further, the present invention relates to methods for preparation of the improved precipitated silica compositions of this invention.
2. Description of the Related Art
The function of an abrasive substance in formulations intended for use in dentifrice compositions is to remove various deposits including pellicle film from the surface of teeth. Pellicle film is tightly adherent and often contains brown or yellow pigments which impart an unsightly appearance to the teeth. While cleaning is important the abrasive should not be so abrasive as to damage the teeth. Thus, an effective dentifrice abrasive material should maximize film removal without causing undue abrasion to the hard tooth tissue. Accordingly, dental researchers are continually seeking dentifrice abrasives that demonstrate satisfactory levels of cleaning without being unduly abrasive and damaging to oral tissues.
Precipitated silica (silicon dioxide) abrasive compositions (also referred to simply as "silicas" herein) and their use in dentifrice formulations are well known. In known silicas, however, there is a nearly linear relation relationship between abrasiveness and cleaning ability. As a result, the ability to develop dentifrice compositions with improved cleaning ability is limited by the tendency of the silicas to become more abrasive as their cleaning properties improve. Thus, there is a need for silicas for use in dentifrice compositions that impart improved cleaning ability to the toothpaste compositions without being overly abrasive and, therefore, damaging to oral tissues.
The cleaning and abrasiveness properties of dentifrice compositions are typically expressed in terms of Pellicle Cleaning Ratios ("PCR") and Radioactive Dentin Abrasion ("RDA") values, respectively. A PCR test measures the ability of a dentifrice composition to remove pellicle film from a tooth under fixed brushing conditions. A PCR test is described generally in "In Vitro Removal of Stain With Dentifrice", G. K. Stookey, T. A. Burkhard and B. R. Schemehorn, J. Dental Research, 61, 1236-9, 1982, which is incorporated herein by reference. The RDA test measures the abrasiveness of dentifrice compositions by measuring the amount of radio-labeled dentin removed from a tooth under fixed brushing conditions. Both PCR and RDA results vary depending upon the nature and concentration of the components of the dentifrice composition.
One key variable affecting PCR and RDA results is the nature and quantity of silica abrasive loaded into the dentifrice composition. It is generally believed that higher concentrations of a given silica abrasive will increase PCR results. Thus, it is desirable to load silica abrasives into dentifrice compositions at relatively high concentrations to improve cleaning. It is also generally believed that relatively hard or abrasive silicas tend to impart relatively high (poor) RDA values to dentifrice compositions. Thus, it is desirable to provide relatively soft or low abrasiveness silicas for use in making dentifrice compositions with low RDA values without sacrificing cleaning properties.
Unfortunately, however, known silicas tend to build viscosity relatively rapidly in dentifrice compositions. As a result most commercial dentifrice compositions contain only approximately 15 to 25% by weight of silica abrasive. Higher loading levels of known silicas tend to make the dentifrice compositions too viscous for their intended purpose. Thus, there is a need for silicas with lower viscosity build characteristics that can be loaded into dentifrice compositions at greater concentrations.
Also, known silicas tend to have a relatively large reactor slurry average particle size (APS). APS is believed to be correlated to abrasivity. While it is possible to dry and mill a silica to almost any desired final, or milled APS, the milling process is itself time consuming and expensive and can have undesired side effects, such as causing discoloration of the silica. More importantly, it is now believed that silicas with lower reactor slurry APS are softer than similar silicas with higher reactor slurry APS. As a result, it is now believed that if two silicas are milled to the same dry milled APS, the silica with the smaller reactor slurry APS will be softer than the silica with the higher reactor slurry APS, will have a lower functional APS and will, therefore, be less abrasive. Accordingly, there is a need for silicas with smaller reactor slurry APS.